Master Logback Filters: Control Log Output with LevelFilter & ThresholdFilter

When you need to control the range of log output in Logback, you can configure different ch.qos.logback.classic.filter.LevelFilter and ch.qos.logback.classic.filter.ThresholdFilter for each appender.

LevelFilter example (ERROR only)

<appender name="ERROR_APPENDER" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <file>logs/error.log</file>
    <filter class="ch.qos.logback.classic.filter.LevelFilter">
        <level>ERROR</level>
        <onMatch>ACCEPT</onMatch>
        <onMismatch>DENY</onMismatch>
    </filter>
    <encoder>
        <pattern>%-4relative [%thread] %-5level %logger{30} - %msg%n</pattern>
    </encoder>
</appender>

The LevelFilter compares the log event's level and accepts only the specified level (here ERROR), denying all others.

ThresholdFilter example (WARN and above)

<appender name="WARN_APPENDER" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <file>logs/warn_error.log</file>
    <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
        <level>WARN</level>
    </filter>
    <encoder>
        <pattern>%-4relative [%thread] %-5level %logger{30} - %msg%n</pattern>
    </encoder>
</appender>

The ThresholdFilter allows all events whose level is at least the specified threshold, so INFO, DEBUG, and TRACE are filtered out.

Custom filter for complex rules

If built‑in filters are insufficient, you can implement your own by extending ch.qos.logback.core.filter.Filter<ILoggingEvent> and overriding the decide method.

public class MyFilter extends Filter<ILoggingEvent> {
    @Override
    public FilterReply decide(ILoggingEvent event) {
        if (event.getLevel() == Level.ERROR) {
            switch (event.getLoggerName()) {
                case "org.springframework.cloud.sleuth.instrument.web.ExceptionLoggingFilter":
                    return FilterReply.DENY;
            }
        }
        return FilterReply.ACCEPT;
    }
}

After implementing the filter, reference it in the appender configuration:

<appender name="WARN_APPENDER" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <file>logs/warn_error.log</file>
    <filter class="com.didispace.log.filter.ExceptionClassFilter"/>
    <encoder>
        <pattern>%-4relative [%thread] %-5level %logger{30} - %msg%n</pattern>
    </encoder>
</appender>

For more details, refer to the official Logback filters documentation.